To clarify the acidic and alkaline Intergranular Stress Corrosion Cracking (IGSCC) mechanism of thermally treated alloy 690 (alloy 690 TT) and shot peened alloy 800 (alloy 800 SP), C-ring tests were conducted in deaerated HCl solutions and in deaerated NaOH solutions at 350°C, compared with the acidic and the alkaline IGSCC susceptibilities of mill-annealed alloy 600 (alloy 600 MA), full-sensitized one (alloy 600 FS) and thermally treated one (alloy 600 TT). Grain boundary characteristics, such as chromium depleted zone and chromium carbide precipitation, were examined using modified Huey test and Transmission Electron Microscopy. Potential-pH diagram for Ni, Cr, Fe-H2O system at 350°C was constructed and the solubilities of NiO, Cr2O3 and Fe3O4 were also calculated to evaluate the stability of oxide films which were formed on the surfaces of alloy 690, 800 and 600.
Under the acidic condition, the IGSCC susceptibility of alloy 800 SP was high. The selective dissolution of Fe and Ni caused by the slip dissolution at the grain boundary could be a contribution factor of acidic IGSCC of alloy 800 SP. For alloy 690 TT, IGSCC was not observed after 15000hr in the C-ring test. Stable oxide film of Cr2O3, which is formed on the surface of alloy 690 TT, and the suppression of slip at the grain boundary due to the chromium carbide formation is responsible for prominent IGSCC resistance.
On the other hand, under the alkaline condition, the IGSCC susceptibility of alloy 800 SP was very low. Stable oxide film formation of Fe3O4 on alloy 800 SP is responsible for prominent IGSCC resistance. For alloy 690 TT, IGSCC did not occur after 10000 hr in the C-ring test. Judging from the test results that the IGSCC susceptibility of alloy 600 MA is very high, and that the one of alloy 600 TT and alloy 690 TT is very low, the suppression of slip at the grain boundary due to the chromium carbide formation is responsible for prominent IGSCC resistance of alloy 690 TT.